Urea-formaldehyde foams have been traditionally made by air-frothing an aqueous suspension or dispersion of urea, formaldehyde, acid catalyst and detergent in water. Frequently, some or all of the formaldehyde and some of the urea is introduced as a condensate wherein the formaldehyde and urea may have partially reacted. Typically, the formaldehyde:urea molar ratio (called in the trade F:U ratio) is greater in the concentrate than in the final product. Thus, the concentrates UF Concentrate-85, a tradename of Allied Chemical Corporation, and "Arboneeld" B, a tradename of E. I. du Pont de Nemours and Company have F:U ratios of, respectively, about 4.8:1 and about 4.1:1. By contrast, most urea-formaldehyde foam products have an F:U ratio between about 1.4:1 and about 2.5:1 and especially between about 1.6:1 and about 2.0:1. These ratios are lowered to the desired levels by adding free urea to the concentrate before or during polymerization.
Such air-frothed urea-formaldehyde foams are generally open-cell in structure and have limited insulative properties and stability. Upon decomposition, such foams emit an unpleasant formaldehyde odor.
There have been suggestions for using low-boiling water insoluble liquids such as the hydrocarbons propane, butane and pentane, several fluorocarbons including dichlorotetrafluoroethane (fluorocarbon 114) and trichloromonofluoromethane (fluorocarbon 11) and other materials such as ethyl chloride, each together with a surface active agent to produce an emulsion with which the polymer precursor components are mixed. Such emulsions generally give product foams which are irregular in cell size and distribution or have generally opened-celled structures. These foams also have insufficient insulative properties and stability for many desired uses.